U.S. patent number 5,523,182 [Application Number 08/333,457] was granted by the patent office on 1996-06-04 for enhanced nickel hydroxide positive electrode materials for alkaline rechargeable electrochemical cells.
This patent grant is currently assigned to Ovonic Battery Company, Inc.. Invention is credited to Peter Benson, Dennis A. Corrigan, Michael A. Fetcenko, Cristian Fierro, Paul R. Gifford, Franklin J. Martin, Stanford R. Ovshinsky.
United States Patent |
5,523,182 |
Ovshinsky , et al. |
June 4, 1996 |
Enhanced nickel hydroxide positive electrode materials for alkaline
rechargeable electrochemical cells
Abstract
A positive electrode material for use in electrochemical cells.
This material comprises particles of positive electrode material
including at least one electrochemically active hydroxide and a
substantially continuous, uniform, encapsulant layer surrounding
the particles of positive electrode material. The encapsulant layer
is formed from a material which, upon oxidation during processing
or during charging of the electrode, is convertible to a highly
conductive form, and which, upon subsequent discharge of the
electrode, does not revert to its previous form. Preferably, the
electrochemically active hydroxide includes at least nickel
hydroxide. The encapsulant layer is preferably formed from at least
cobalt hydroxide or cobalt oxyhydroxide. This layer is formed on
the particles of positive electrode material by precipitation from
a cobalt salt solution, which can be a cobalt sulfate solution.
Also disclosed are positive electrodes including the material and a
precipitation method of forming the material.
Inventors: |
Ovshinsky; Stanford R.
(Bloomfield Hills, MI), Fetcenko; Michael A. (Rochester
Hills, MI), Fierro; Cristian (Troy, MI), Gifford; Paul
R. (Troy, MI), Corrigan; Dennis A. (Troy, MI),
Benson; Peter (Rochester, MI), Martin; Franklin J.
(Rochester Hills, MI) |
Assignee: |
Ovonic Battery Company, Inc.
(Troy, MI)
|
Family
ID: |
23302879 |
Appl.
No.: |
08/333,457 |
Filed: |
November 2, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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300610 |
Sep 2, 1994 |
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308764 |
Sep 19, 1994 |
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27973 |
Mar 8, 1993 |
5348822 |
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975031 |
Nov 12, 1992 |
5344728 |
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Current U.S.
Class: |
429/223;
429/224 |
Current CPC
Class: |
H01M
4/624 (20130101); H01M 4/32 (20130101); H01M
4/52 (20130101); H01M 4/366 (20130101); H01M
2300/0014 (20130101); Y02E 60/10 (20130101); H01M
10/30 (20130101) |
Current International
Class: |
H01M
10/34 (20060101); H01M 4/28 (20060101); H01M
4/26 (20060101); H01M 10/24 (20060101); H01M
4/70 (20060101); H01M 4/32 (20060101); H01M
10/26 (20060101); H01M 4/52 (20060101); H01M
4/80 (20060101); H01M 004/32 (); H01M 004/52 () |
Field of
Search: |
;429/218,223,224 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
F A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 3rd
edition, Interscience Publishers, 1972, pp. 189, 208, 802..
|
Primary Examiner: Willis, Jr.; Prince
Assistant Examiner: Chaney; Carol
Attorney, Agent or Firm: Luddy; Marc J. Siskind; Marvin S.
Schumaker; David W.
Parent Case Text
CONTINUING INFORMATION
This application is a continuation-in-part of U.S. patent
application Ser. No. 08/300,610 (filed 2 Sep. 1994) and U.S. patent
application Ser. No. 08/308,764 (filed 19 Sep. 1994) both of which
are continuations in part of U.S. Pat. No. 5,348,822 (application
Ser. No. 08/027,973, filed 8 Mar. 1993) which is a continuation in
part of U.S. Pat. No. 5,344,728 (application Ser. No. 07/975,031,
filed 12 Nov. 1992).
Claims
We claim:
1. A positive electrode material for the formation of a paste for
fabricating positive electrodes comprising:
particles of a nickel hydroxide positive electrode material;
and
a precursor coating of a substantially continuous, uniform
encapsulant layer precipitated on said particles prior to
preparation of active material paste, said encapsulant layer formed
from a material that upon oxidation during processing or during
charging increases resistance to corrosion products, increases the
conductivity of said particles, and does not revert to its
precharge form upon subsequent discharge;
where said nickel hydroxide additionally includes
at least three compositional modifiers chosen from the group
consisting of Al, Bi, Co, Cr, Cu, Fe, In La Mn, Ru, Sb, Ti, and Zn
and
at least one chemical modifier chosen from the group consisting of
Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn.
2. The positive electrode material of claim 1, wherein said
encapsulant layer is formed from at least one component chosen from
the group consisting of cobalt hydroxide, cobalt oxyhydroxide,
manganese hydroxide, and a manganese oxide.
3. The positive electrode material of claim 2, wherein said
encapsulant layer of is formed upon said particles of positive
electrode material by precipitation from a salt solution.
4. The positive electrode material of claim 3, wherein said salt
solution is a cobalt sulfate solution.
5. An electrochemical storage cell comprising:
at least one positive electrode;
at least one negative electrode; and
electrolyte;
where said at least one positive electrode is a positive electrode
formed from a paste of particles of a nickel hydroxide positive
electrode material having
a precursor coating of a substantially continuous, uniform
encapsulant layer preciptated on said particles prior to
preparation of active material paste, said encapsulant layer formed
from a material that upon oxidation during processing or during
charging increases resistance to corrosion products, increases the
conductivity of said particles, and does not revert to its
precharge form upon subsequent discharge;
where said nickel hydroxide additionally includes
at least three compositional modifiers chosen from the group
consisting of Al, Bi, Co, Cr, Cu, Fe, In, La, Mn, Ru, Sb, Sn, Ti,
and Zn and
at least one chemical modifier chosen from the group consisting of
Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn.
6. The electrochemical storage cell of claim 5, wherein said at
least one electrochemically active hydroxide includes at least
nickel hydroxide.
7. The electrochemical storage cell of claim 5, wherein said
encapsulant layer is formed from at least one component chosen from
the group consisting of cobalt hydroxide, cobalt oxyhydroxide,
manganese hydroxide, and a manganese oxide.
8. The electrochemical storage cell of claim 7, wherein said
encapsulant layer is formed by precipitation from a salt
solution.
9. The electrochemical storage cell of claim 8, wherein said salt
solution is a cobalt sulfate solution.
10. A method of making a positive electrode material for the
formation of a paste for fabricating positive electrodes for use in
an electrochemical cell, comprising the steps of
forming particles of a nickel hydroxide positive electrode
material;
precipitating a precursor coating of a substantially continuous,
uniform encapsulant layer on said particles prior to preparation of
active material paste, said encapsulant layer formed from a
material that upon charging increases resistance to corrosion
products and increases the conductivity of said particles, and does
not revert to its precharge form upon subsequent discharge;
where said nickel hydroxide additionally includes
at least three compositional modifiers chosen from the group
consisting of Al, Bi, Co, Cr, Cu, Fe, In, La, Mn, Ru, Sb, Sn, Ti,
and Zn and
at least one chemical modifier chosen from the group consisting of
Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn.
11. The method of claim 10, wherein said encapsulant layer is
formed from at least one component chosen from the group consisting
of cobalt hydroxide, cobalt oxyhydroxide, manganese hydroxide, and
a manganese oxide.
12. The method of claim 11, wherein said precipitation of said
encapsulant occurs from a salt solution and further comprises the
step of:
converting said encapsulating layer from a hydroxide to an
oxyhydroxide using air oxidation.
13. The method of claim 12, wherein said salt solution is a cobalt
sulfate solution.
14. A nickel metal hydride battery exhibiting an insignificant
increase in internal pressure during cycling and a cycle life
.gtoreq.500 cycles, said nickel metal hydride battery
comprising:
a pasted positive electrode formed from particles of nickel
hydroxide positive electrode material; and
a precursor coating of a substantially continuous, uniform
encapsulant layer precipitated on said particles prior to
preparation of active material paste, said encapsulant layer formed
from a material that upon oxidation during processing or during
charging increases resistance to corrosion products, increases the
conductivity of said particles, and does not revert to its
precharge form upon subsequent discharge;
where said nickel hydroxide additionally includes
at least three compositional modifiers chosen from the group
consisting of Al, Bi, Co, Cr, Cu, Fe, In, La, Mn, Ru, Sb, Sn, Ti,
and Zn and
at least one chemical modifier chosen from the group consisting of
Al, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn.
Description
FIELD OF THE INVENTION
The present invention relates generally to an optimized nickel
hydroxide positive electrode materials. More specifically, this
invention relates to an enhanced nickel hydroxide positive
electrode where particles of nickel hydroxide material are
encapsulated with a conductive material. The encapsulant layer is
formed from a material which, upon oxidation during processing or
during charging of the electrode, is convertible to an electrically
conductive form, and which, upon discharge of the electrode, does
not revert to its previous form. Cobalt oxide and hydroxide of
various oxidation states is the preferred material for
encapsulation. Cells using the present invention have demonstrated
a remarkable reduction in internal pressure rise during cycling.
The instant invention also relates to a method for forming the
modified nickel hydroxide powder materials.
BACKGROUND OF THE INVENTION
In rechargeable alkaline cells, weight and portability are
important considerations. It is also advantageous for rechargeable
alkaline cells to have long operating lives without the necessity
of periodic maintenance. Rechargeable alkaline cells are used in
numerous consumer devices such as calculators, portable radios, and
cellular phones. They are often configured into a sealed power pack
that is designed as an integral part of a specific device.
Rechargeable alkaline cells can also be configured as larger cells
that can be used, for example, in industrial, aerospace, and
electric vehicle applications.
The best rechargeable alkaline cells are ones that can operate as
an "install and forget" power source. With the exception of
periodic charging, a rechargeable alkaline cell should perform
without attention and should not become a limiting factor in the
life of the device it powers.
Stanford R. Ovshinsky, by applying his fundamental principles of
disorder, pioneered the development of the first commercial nickel
metal hydride (NiMH) battery. For more than three decades,
virtually every other manufacturer in the world studied the NiMH
battery technology, but no commercial battery of this kind existed
until after the publication of U.S. Pat. No. 4,623,597 to Ovshinsky
and Ovshinsky's related technical papers which disclosed basic and
fundamentally new principles of battery material design. NiMH
batteries are the only truly "green" battery because they can be
completely recycled. NiMH batteries are the only rechargeable
battery that can meet society's requirements for an ecological,
renewable source of electrochemical energy.
Ni--MH cells utilize a negative electrode that is capable of the
reversible electrochemical storage of hydrogen. Ni--MH cells
usually employ a positive electrode of nickel hydroxide material.
The negative and positive electrodes are spaced apart in the
alkaline electrolyte. Upon application of an electrical potential
across a Ni-MH cell, the Ni-MH material of the negative electrode
is charged by the electrochemical absorption of hydrogen and the
electrochemical discharge of a hydroxyl ion, as shown in equation
(1): ##STR1## The negative electrode reactions are reversible. Upon
discharge, the stored hydrogen is released to form a water molecule
and release an electron. The reactions that take place at the
nickel hydroxide positive electrode of a Ni--MH cell are shown in
equation (2): ##STR2## Ni--MH materials are discussed in detail in
U.S. Pat. No. 5,277,999 to Ovshinsky, et al., the contents of which
are incorporated by reference.
As previously mentioned, Stanford R. Ovshinsky was responsible for
inventing new and fundamentally different electrochemical electrode
materials. As predicted by Ovshinsky, detailed investigation by
Ovshinsky's team determined that reliance on simple, relatively
pure compounds was a major shortcoming of the prior art. Relatively
pure crystalline compounds were found to have a low density of
hydrogen storage sites, and the type of sites available occurred
accidently and were not designed into the bulk of the material.
Thus, the efficiency of the storage of hydrogen and the subsequent
release of hydrogen to form water was determined to be poor. By
applying his fundamental principles of disorder to electrochemical
hydrogen storage, Ovshinsky drastically departed from conventional
scientific thinking and created a disordered material having an
ordered local environment where the entire bulk of the material was
provided with catalytically active hydrogen storage sites.
Short-range, or local, order is elaborated on in U.S. Pat. No.
4,520,039 to Ovshinsky, entitled Compositionally Varied Materials
and Method for Synthesizing the Materials, the contents of which
are incorporated by reference. This patent discusses how disordered
materials do not require any periodic local order and how, by using
Ovshinsky's techniques, spatial and orientational placement of
similar or dissimilar atoms or groups of atoms is possible with
such increased precision and control of the local configurations
that it is possible to produce qualitatively new phenomena. In
addition, this patent discusses that the atoms used need not be
restricted to "d band" or "f band" atoms, but can be any atom in
which the controlled aspects of the interaction with the local
environment and/or orbital overlap plays a significant role
physically, electronically, or chemically so as to affect physical
properties and hence the functions of the materials. Ovshinsky's
use of disordered materials has fundamental scientific advantages.
The elements of these materials offer a variety of bonding
possibilities due to the multidirectionality of d-orbitals. The
multidirectionality ("porcupine effect") of d-orbitals provides for
a tremendous increase in density and hence active storage sites.
These techniques result in means of synthesizing new materials
which are disordered in several different senses
simultaneously.
Ovshinsky had previously found that the number of surface sites
could be significantly increased by making an amorphous film that
resembled the surface of the desired relatively pure materials. As
Ovshinsky explained in Principles and Applications of Amorphicity,
Structural Change, and Optical Information Encoding, 42 Journal De
Physique at C4-1096 (October 1981):
Amorphicity is a generic term referring to lack of X-ray
diffraction evidence of long-range periodicity and is not a
sufficient description of a material. To understand amorphous
materials, there are several important factors to be considered:
the type of chemical bonding, the number of bonds generated by the
local order, that is its coordination, and the influence of the
entire local environment, both chemical and geometrical, upon the
resulting varied configurations. Amorphicity is not determined by
random packing of atoms viewed as hard spheres nor is the amorphous
solid merely a host with atoms imbedded at random. Amorphous
materials should be viewed as being composed of an interactive
matrix whose electronic configurations are generated by free energy
forces and they can be specifically defined by the chemical nature
and coordination of the constituent atoms. Utilizing multi-orbital
elements and various preparation techniques, one can outwit the
normal relaxations that reflect equilibrium conditions and, due to
the three-dimensional freedom of the amorphous state, make entirely
new types of amorphous materials--chemically modified
materials.
Once amorphicity was understood as a means of introducing surface
sites in a film, it was possible to produce "disorder" that takes
into account the entire spectrum of local order effects such as
porosity, topology, crystallites, characteristics of sites, and
distances between sites. Thus, rather than searching for material
modifications that would yield ordered materials having a maximum
number of accidently occurring surface irregularities, Ovshinky's
team at ECD began constructing "disordered" materials where the
desired irregularities were tailor made. See, U.S. Pat. No.
4,623,597, the disclosure of which is incorporated by
reference.
The term "disordered," as used herein corresponds to the meaning of
the term as used in the literature, such as the following:
A disordered semiconductor can exist in several structural states,
This structural factor constitutes a new variable with which the
physical properties of the [material] . . . can be controlled.
Furthermore, structural disorder opens up the possibility to
prepare in a roetastable state new compositions and mixtures that
far exceed the limits of thermodynamic equilibrium. Hence, we note
the following as a further distinguishing feature. In many
disordered [materials] . . . it is possible to control the
short-range order parameter and thereby achieve drastic changes in
the physical properties of these materials, including forcing new
coordination numbers for elements . . . .
S. R. Ovshinsky, The Shape of Disorder, 32 Journal of
Non-Crystafiine Solids at 22 (1979) (emphasis added).
The "short-range order" of these disordered materials are further
explained by Ovshinsky in The Chemical Basis of Amorphicity:
Structure and Function, 26:8-9 Rev. Roum. Phys. at 893-903
(1981):
[S] hort-range order is not conserved . . . . Indeed, when
crystalline symmetry is destroyed, it becomes impossible to retain
the same short-range order. The reason for this is that the
short-range order is controlled by the force fields of the electron
orbitals therefore the environment must be fundamentally different
in corresponding crystalline and amorphous solids. In other words,
it is the interaction of the local chemical bonds with their
surrounding environment which determines the electrical, chemical,
and physical properties of the material, and these can never be the
same in amorphous materials as they are in crystalline materials .
. . . The orbital relationships that can exist in three-dimensional
space in amorphous but not crystalline materials are the basis for
new geometries, many of which are inherently anti-crystalline in
nature. Distortion of bonds and displacement of atoms can be an
adequate reason to cause amorphicity in single component materials.
But to sufficiently understand the amorphicity, one must understand
the three-dimensional relationships inherent in the amorphous
state, for it is they which generate internal topology incompatible
with the translational symmetry of the crystalline lattice . . . .
What is important in the amorphous state is the fact that one can
make an infinity of materials that do not have any crystalline
counterparts, and that even the ones that do are similar primarily
in chemical composition. The spatial and energetic relationships of
these atoms can be entirely different in the amorphous and
crystalline forms, even though their chemical elements can be the
same . . .
Short-range, or local, order is elaborated on in U.S. Pat. No.
4,520,039 to Ovshinsky, entitled Compositionally Varied Materials
and Method for Synthesizing the Materials, the contents of which
are incorporated by reference. This patent discusses how disordered
materials do not require any periodic local order and how, by using
Ovshinsky's techniques, spatial and orientational placement of
similar or dissimilar atoms or groups of atoms is possible with
such increased precision and control of the local configurations
that it is possible to produce qualitatively new phenomena. In
addition, this patent discusses that the atoms used need not be
restricted to "d band" or "f band" atoms, but can be any atom in
which the controlled aspects of the interaction with the local
environment plays a significant role physically, electrically, or
chemically so as to affect the physical properties and hence the
functions of the materials. These techniques result in means of
synthesizing new materials which are disordered in several
different senses simultaneously.
By forming metal hydride alloys from such disordered materials,
Ovshinsky and his team were able to greatly increase the reversible
hydrogen storage characteristics required for efficient and
economical battery applications, and produce, for the first time,
commercially viable batteries having high density energy storage,
efficient reversibility, high electrical efficiency, bulk hydrogen
storage without structural change or poisoning, long cycle life,
and deep discharge capability.
The improved characteristics of these alloys result from tailoring
the local chemical order and hence the local structural order by
the incorporation of selected modifier elements into a host matrix.
Disordered metal hydride alloys have a substantially increased
density of catalytically active sites and storage sites compared to
conventional ordered materials. These additional sites are
responsible for improved efficiency of electrochemical
charging/discharging and an increase in electrical energy storage
capacity. The nature and number of storage sites can even be
designed independently of the catalytically active sites. More
specifically, these disordered multi-component alloys are
thermodynamically tailored to allow storage of hydrogen atoms at a
wide range of modulated bonding strengths within the range of
reversibility suitable for use in secondary battery
applications.
Based on these principles of disordered materials, described above,
a family of extremely efficient electrochemical hydrogen storage
materials were formulated. These are the Ti--V--Zr--Ni type active
materials such as disclosed by Ovshinsky's team in U.S. Patent No.
4,551,400 ("the '400 Patent"), the disclosure of which is
incorporated by reference. These materials reversibly form hydrides
in order to store hydrogen. All the materials used in the '400
Patent utilize a Ti--V--Ni composition, where at least Ti, V, and
Ni are present with at least one or more of Cr, Zr, and AI. The
materials of the '400 Patent are generally multiphase
polycrystalline materials, which may contain, but are not limited
to, one or more phases of Ti--V--Zr--Ni material with C.sub.14 and
C.sub.15 type crystal structures. Other Ti--V--Zr--Ni alloys may
also be used for fabricating rechargeable hydrogen storage negative
electrodes. One such family of materials are those described in
U.S. Pat. No. 4,728,586 ("the '586 Patent"), titled Enhanced Charge
Retention Electrochemical Hydrogen Storage Afioys and an Enhanced
Charge Retention Electrochemical Cell, the disclosure of which is
incorporated by reference.
The characteristic surface roughness of the metal electrolyte
interface is a result of the disordered nature of the material.
Since all of the constituent elements, as well as many alloys and
phases of them, are present throughout the metal, they are also
represented at the surfaces and at cracks which form in the
metal/electrolyte interface. Thus, the characteristic surface
roughness is descriptive of the interaction of the physical and
chemical properties of the host metals as well as of the alloys and
crystallographic phases of the alloys, in an alkaline environment.
The microscopic chemical, physical, and crystallographic parameters
of the individual phases within the hydrogen storage alloy material
are believed to be important in determining its macroscopic
electrochemical characteristics.
In addition to the physical nature of its roughened surface, it has
been observed that V--Ti--Zr--Ni alloys tend to reach a steady
state surface condition and particle size. This steady state
surface condition is characterized by a relatively high
concentration of metallic nickel. These observations are consistent
with a relatively high rate of removal through precipitation of the
oxides of titanium and zirconium from the surface and a much lower
rate of nickel solubilization. The resultant surface seems to have
a higher concentration of nickel than would be expected from the
bulk composition of the negative hydrogen storage electrode. Nickel
in the metallic state is electrically conductive and catalytic,
imparting these properties to the surface. As a result, the surface
of the negative hydrogen storage electrode is more catalytic and
conductive than if the surface contained a higher concentration of
insulating oxides.
The surface of the negative electrode, which has a conductive and
catalytic component--the metallic nickel--appears to interact with
chromium alloys in catalyzing various hydride and dehydride
reaction steps. To a large extent, many electrode processes,
including competing electrode processes, are controlled by the
presence of chromium in the hydrogen storage alloy material, as
disclosed in the '586 Patent.
Rechargeable alkaline cells can be either vented cells or sealed
cells. During normal operation, a vented cell typically permits
venting of gas to relieve excess pressure as part of the normal
operating behavior. In contrast, a sealed cell generally does not
permit venting on a regular basis. As a result of this difference,
the vent assemblies and the amounts of electrolyte in the cell
container relative to the electrode geometry both differ
significantly.
Vented cells operate in a "flooded condition." The term "flooded
condition" means that the electrodes are completely immersed in,
covered by, and wetted by the electrolyte. Thus, such cells are
sometimes referred to as "flooded cells." A vented cell is
typically designed for very low operating pressures of only a few
pounds per square inch after which excess pressures are relieved by
a vent mechanism.
In contrast, sealed cells are designed to operate in a "starved"
electrolyte configuration, that is with only the minimum amount of
electrolyte necessary to permit gas recombination. The enclosure
for a sealed cell is normally metallic and the cell may be designed
for operation at up to about 100 p.s.i. absolute or higher. Because
they are sealed, such cells do not require periodic
maintenance.
Typically, a sealed rechargeable alkaline cell for use in consumer
appliances, such as a C cell, uses a cylindrical nickel-plated
steel case as the negative terminal and the cell cover as the
positive terminal. An insulator separates the positive cover from
the negative cell can. The electrodes are wound to form a compact
"jelly roll" with the electrodes of opposite polarity isolated from
each other by a porous, woven or non-woven separator of nylon or
polypropylene, for example. A tab extends from each electrode to
create a single current path through which current is distributed
to the entire electrode area during charging and discharging. The
tab on each electrode is electrically connected to its respective
terminal.
In sealed cells, the discharge capacity of a nickel based positive
electrode is limited by the amount of electrolyte, the amount of
active material, and the charging efficiencies. The charge
capacities of a Cd negative electrode and a MH negative electrode
are both provided in excess, to maintain the optimum capacity and
provide overcharge protection.
An additional goal in making any type of electrode is to obtain as
high an energy density as possible. For small batteries, the volume
of a nickel hydroxide positive electrode is sometimes more
important than weight, and the volumetric capacity is usually
measured in mAh/cc, or an equivalent units and specific capacity is
written as mAh/g.
At present, sintered, foamed, or pasted nickel hydroxide positive
electrodes are used in NiCd and Ni--MH cells. The process of making
sintered electrodes is well known in the art. Conventional sintered
electrodes normally have an energy density of around 480-500
mAh/cc. In order to achieve significantly higher loading, the
current trend has been away from sintered positive electrodes and
toward foamed and pasted electrodes.
In general, sintered positive electrodes are constructed by
applying a nickel powder slurry to a nickel-plated steel base
followed by sintering at high temperature. This process causes the
individual particles of nickel to weld at their points of contact
resulting in a porous material that is approximately 80% open
volume and 20% solid metal. This sintered material is then
impregnated with active material by soaking it in an acidic
solution of nickel nitrate, followed by conversion to nickel
hydroxide by reaction with an alkali metal hydroxide. After
impregnation, the material is subjected to electrochemical
formation.
In practice, electrode capacity beyond the one-electron transfer
theoretical capacity is not usually observed. One reason for this
is incomplete utilization of the active material due to electronic
isolation of oxidized material. Because reduced nickel hydroxide
material has a high electronic resistance, the reduction to nickel
hydroxide adjacent the current collector forms a less conductive
surface that interferes with the subsequent reduction of oxidized
active that is farther away. Ovshinsky and his team have developed
positive electrode materials that have demonstrated reliable
transfer of more than one electron per nickel atom. Such materials
are described in U.S. Pat. No. 5,344,728 and 5,348,822 (which
describe stabilized disordered positive electrode materials) and
copending U.S. patent application Ser. No. 08/300,610 filed Aug.
23, 1994, and U.S. patent application Ser. No. 08/308,764 filed
Sep. 19, 1994.
It is known in the art that cobalt metal,cobalt hydroxide, and
cobalt oxide can be added to nickel hydroxide at a level of
typically 0-5% in commercial applications. This level of cobalt
additive is used to improve capacity and rate capability, provide
precharge to the negative electrode by dissolving and
redistributing as an electrically conductive network within the
nickel hydroxide active material.
It has been postulated that when Co and CoO additives are mixed
with nickel hydroxide powder to form a paste for the production of
pasted electrodes that Co and CoO form an interconnected network of
cobalt hydroxide (Co(OH).sub.2 CoOOH) that increases electronic
conductivity. This in turn would result in higher electrode
capacities and utilizations.
This conductivity increase is thought to occur for the reasons
described in U.S. Pat. No. 4,844,999 to Oshitani, et al. and U.S.
Pat. No. 4,985,318 also to Oshitani, et al. both of which are
hereby incorporated by reference. Both of these patents describe
mixing cobalt compound powders with nickel hydroxide active
material to form a paste. It is thought that improvements resulting
from the addition of cobalt compounds occurs because cobalt
increases the conductivity between the nickel hydroxide particles
themselves.
In that respect, Oshitani, et al., in U.S. Pat. No. 4,844,999 (the
'999 patent) teach that:
"When a cobalt compound additive is dissolved outside the crystals
of nickel hydroxide to establish connection between the current
collector and the nickel hydroxide particles by virtue of the
reaction,
before the battery is charged, the cobalt compound is converted
into cobalt oxyhydroxide of high conductivity by virtue of the
reaction,
to smoothen [sic] the flow of electrons between the nickel fibers
of the current collector and the nickel hydroxide particles and
increase the ratio of utilization of the active material. The
mechanism of the reactions mentioned above is depicted in the model
in FIG. 1." (Reproduced as FIG. 1 of the present application.)
The '999 patent also states:
"For the active material to react, smooth passage of electrons from
the current collector to the surface of the particles of active
material is an essential requirement. To the smooth [sic] passage
of electrons, the presence of an electroconductive network of CoOOH
particles in a free state (existing in the surface of the CoOOH
[sic] particles without forming a solid solution with nickel
hydroxide) is indispensable.
As regards the CoO additive which is destined to form the network,
FIG. 7 (reproduced as FIG. 2 in the present application) shows the
relation between the amount of CoO added, the ratio of utilization
of active material, and the energy density per unit volume of the
electrode sheet. As the amount of the CoO added is increased, so
the ratio of utilization of active material is heightened to be
converged in the vicinity of 100%. Since the additive itself merely
contributes to the electroconductivity and takes no part actually
in discharge, the actual energy density of the electrode sheet
tends to decline from the vicinity of 15%."
Finally, the '999 patent states:
"The pasted electrode loaded with the powder in which nickel
hydroxide is dipped in HCoO.sub.2 - ions to form a cobalt hydroxide
layer on the surface was inferior to the electrode formed by mixing
CoO powders in respect of the ratio of utilization of active
material and was as much as the electrode formed by mixing
.beta.-Co(OH).sub.2 powders in the ratio of utilization of active
material. As for the pasted electrode loaded with the powder in
which a conductive CoOOH layer is formed on the surface of nickel
oxyhydroxide (the powder of which obtained [sic] by removing nickel
fiber which is a current collector from the electrode formed by
mixing CoO powders after charging and discharging the electrode),
the ratio of utilization of active material was inferior. This
teaches that it is indispensably required to form a conductive
network (CoOOH) of active material and current collector in the
produced electrode and that the formation of the conductive network
in advance on the surface of the active material provides an
insufficient effect."
Thus, the '999 patent describes how Oshitani, et al., were not able
to produce electrodes that were simultaneously capable of a high
active material utilization and a high energy density (which relate
directly to electrode capacity). In addition, the '999 patent fails
to teach or suggest the role of the CoOOH network in inhibiting
oxygen evolution and in protecting against potential poisons that
can promote premature oxygen evolution. In fact, the '999 patent
describes how prior art electrodes were known to routinely develop
"dead spots," or areas of nickel hydroxide active material lacking
cobalt coating that were prone to poisoning.
Unfortunately, the addition of compounds as described in the '999
patent, decreases the Ni(OH).sub.2 content from 90 wt % in the
nickel hydroxide powder to approximately 80 wt % in the pasted
material. This results in adverse effects on electrode capacity
since cobalt compounds are not electrochemically active.
Thus, there is a need in the art for an active material
powder/electrode system which can deliver both very high active
material utilization and high electrode capacity.
SUMMARY OF THE INVENTION
The main objectives of the present invention is to provide a more
uniform distribution of the cobalt network, inhibit gas evolution,
and provide resistance to corrosion products for internal pressure
reduction and stability.
Another objective of the present invention is to provide an
electrode material that contains a decreased amount of CoO and
Co(OH).sub.2 in the electrode preparation that simultaneously
delivers the same, or increased utilization without a decrease in
specific capacity.
Another objective of the present invention is to attain the
characteristics described above at a decreased fabrication cost
compared to the prior art.
These and other objectives are achieved by the positive electrode
material of the present invention. This material comprises positive
electrode particles including at least one electrochemically active
hydroxide surrounded by a substantially continuous, uniform,
encapsulant layer. This encapsulant layer is formed from a material
that upon oxidation during processing or during charging becomes
conductive, and does not revert to its precharge form upon
subsequent discharge. Preferably, the electrochemically active
hydroxide includes at least nickel hydroxide, and most preferably a
Ni/Co/Zn triprecipitate.
The encapsulant layer is preferably formed from at least cobalt
hydroxide, cobalt oxyhydroxide, manganese hydroxide, or a manganese
oxide. This encapsulant layer is formed on the positive electrode
particles by precipitation from a salt solution. An example of a
cobalt salt solution is a cobalt sulfate solution. A particularly
useful and stable form of encapsulant layer is attained by air
oxidation of the cobalt hydroxide immediately following
precipitation.
The nickel hydroxide used in the present invention can additionally
includes at least one compositional modifier chosen from the group
consisting of AI, Bi, Co, Cr, Cu, Fe, In, La, Mn, Ru, Sb, Sn, Ti,
and Zn or one chemical modifier chosen from the group consisting of
AI, Ba, Ca, Co, Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn.
The present invention further provides for an electrochemical
storage cell comprising: at least one positive electrode; at least
one negative electrode; and electrolyte. The positive electrode
includes the electrochemical storage material of the present
invention.
Finally, the present invention includes a precipitation method for
forming the encapsulant layer upon the particles of
electrochemically active hydroxide material.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a simplistic one-dimensional view of the method of
the prior art where cobalt is incorporated into nickel hydroxide
positive electrodes to enhance their performance.
FIG. 2 is a graphical representation of data of the prior art data
taken on positive electrodes that incorporate CoO. This figure
specifically plots the percentage of CoO added to the electrode
paste material on the ordinate versus both the percentage of active
material utilization and energy density of the electrode (in
mAh/cc) on the abscissa.
FIG. 3 is a schematic view of a cut-away portion of a three
dimensional electrode which more realistically depicts the
relationships between the components of the prior art and
electrodes that contain CoO.
FIG. 4 depicts the prior art electrode of FIG. 3 after the CoO has
dissolved in the electrolyte solution and has deposited cobalt
hydroxide onto the electrode materials. Specifically shown are the
non-coated, "dead" areas of the electrode;
FIG. 5 depicts a similar electrode of the present invention,
specifically pointing out the uniform, continuous coating of cobalt
hydroxide which is pre-formed on the nickel hydroxide particles,
thus eliminating "dead" spots.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a simplistic prior art view from U.S. Pat. 4,844,999
(discussed above), that illustrates how the addition of cobalt
compounds, such as cobalt or cobalt oxide, result in the formation
of a conductive network around nickel hydroxide active material
particles in pasted electrodes. The cobalt compounds are dissolved
in the electrolyte of the battery and are then redeposited onto the
nickel hydroxide as cobalt hydroxide. During charging, the cobalt
hydroxide is converted, in a non-reversible reaction, to cobalt
oxyhydroxide. Cobalt oxyhydroxide is a highly conductive material
which assists in cycling the active material by allowing electrons
to flow more readily from the active material to the nickel fiber
or foam substrate.
Unfortunately, the interconnected network described in the prior
art is discontinuous. The prior art acknowledges that at least some
areas of the active material do not participate in the
charge/discharge cycling of the battery. This is most clearly seen
in the prior art FIG. 2. This figure plots the percentage of CoO
added to the electrode paste material on the ordinate versus both
the percentage of active material utilization and energy density of
the electrode (in mAh/cc) on the abscissa. FIG. 2 shows that to
achieve higher energy densities, the amount of CoO added must be
kept at a point which does not allow for optimal active material
utilization. This is because as more and more CoO is added, the
percentage of active nickel hydroxide in the electrode is reduced.
Thus, as more CoO is added, while the percentage utilization of the
active material is increased, the energy density of the electrode
is decreased. There is also no guarantee that the cobalt network
even with sufficient CoO additive remains uniform and sufficient
after long term storage, cycling, or upon exposure to elevated
temperatures.
This dichotomy exists because an electrode is not as simplistic and
one dimensional as is FIG. 1 depicts. FIG. 3 is a schematic view of
a cut-away of a three dimensional electrode which more
realistically depicts the relationships between the components of
the electrode.
Specifically, FIG. 3 shows particles of nickel hydroxide active
material 1 intermixed with particles of a cobalt compound 2, such
as cobalt oxide. These particles are pasted on a substrate 3 of
nickel fiber matte or nickel foam. This substrate 3 provides the
electrical connection for the active material as well as structural
support.
It is well known that the cobalt compound dissolves into the
electrolyte, is deposited as cobalt hydroxide onto the active
material (and everywhere else), and is converted into cobalt
oxyhydroxide in the electrode during charging. However, the
resulting cobalt hydroxide coating is not as uniform and continuous
as is depicted in FIG. 1. In reality, as shown in FIG. 4, while
there are many areas 4 where cobalt hydroxide is deposited onto the
nickel hydroxide active material particles 1, there are also just
as many areas 5 where cobalt hydroxide is not deposited.
The areas 5, where there is no deposition, are critical. Without
wishing to be bound by theory, it is believed that the areas 5
where there is no deposition are areas where the nickel hydroxide
particles are too tightly packed to be accessible to dissolved
cobalt; areas of intercontact between nickel hydroxide particles,
and areas of contact between nickel hydroxide particles and the
substrate. These areas are critical because without proper
electrical connection, some portions of the nickel hydroxide active
material will be left isolated from the electrical network of the
electrode--and therefore under utilized. These areas are also
susceptible to corrosion products from both the separator and metal
hydride alloy. Additionally, a portion of the cobalt added to the
electrode according to the prior art is wasted by deposition onto
unnecessary portions of the electrode such as non-interconnected
portions of the nickel fibers 3, as well as other parts of the
batten such as can walls, electrode tabs, and separator.
These problems are overcome by the staged cobalt dips described in
U.S. Pat. No. 5,394,728, (the '728 patent) of which the present
application is a continuation in part. The contents of this patent,
which describe a sintered nickel hydroxide positive electrode, are
specifically incorporated by reference. As discussed in the '728
patent, the use of cobalt dips during the impregnation of the
electrode results in enriched cobalt surface which provides better
conductivity, poisoning resistance, and suppressed O.sub.2
evolution.
These interconnect and waste problems are not overcome by in the
prior art directed toward pasted electrode materials. This body of
prior art teaches the addition of excess cobalt. As noted above,
this leads to decreased energy density because as more cobalt
(which does not contribute to the electrochemical capacity of the
electrode) is added, the amount, and thus the percentage, of the
nickel hydroxide active material in the electrode decreases.
The present invention describes a pasted positive electrode
utilizing the principles of cobalt coating described in the '728
patent. The present application builds on these principles, and
describes pasted positive electrode materials that overcome the
deficiencies of the of the prior art pasted materials. FIG. 5
illustrates the significant difference that exists between the
prior art of pasted materials and the present invention. In the
present invention, instead of adding cobalt to the electrode paste
and allowing an in-situ, haphazard formation of a discontinuous,
non-uniform cobalt hydroxide network, a continuous, uniform coating
of cobalt hydroxide 4 is pre-formed in a manner analogous to that
described in the '728 patent onto the nickel hydroxide active
material particles before their incorporation into the electrode.
This insures that the areas of inter-particle contact, as well as,
the areas of contact between the active material and the substrate
contain the required cobalt oxyhydroxide conductive material. (This
cobalt oxyhydroxide is formed upon initial charging of the
electrode from the pre-deposited cobalt hydroxide).
The present invention allows for a reduction in the total cobalt
used in the electrode. This allows for a greater percentage of
nickel hydroxide active material to be used in the electrode. Thus,
a high percentage utilization of the nickel hydroxide material is
achieved because there is good electrical interconnection between
particles and between the particles and the substrate.
Additionally, increased energy density is also achieved because the
amount of cobalt used to achieve the interconnection is reduced
allowing for more active material to be incorporated.
EXAMPLES
The capacity and utilization of prior art type comparative pasted
electrodes using cobalt metal and cobalt oxide additives to the
nickel hydroxide was first measured using small tri-electrode
cells. In the first set of experiments the formation of the
positive was similar to the process used on EV cells, described
below. The utilization in this case was found to be above 90% with
the gravimetric and volumetric energy densities at 176 mAh/g and
568 mAh/cc respectively. In a second set of experiments the
electrodes were electrically formed at C10. In this case the
utilization was found to similar or slightly better than the
previous case.
A small tri-electrode cell for testing pasted electrodes was
designed. The cell was fabricated by positioning a positive
electrode (3 in.sup.2 area) between two negatives and then placing
the three electrode system inside a plastic bag in the presence of
excess electrolyte. The negative electrodes were activated in 30%
KOH at 75 .degree. C. for 3 hrs prior to assembling the cell. The
three electrode system was then held in place by two plexiglass
plates.
Ni-MH negative electrodes were prepared as described in copending
U.S. patent application Ser. No. 08/027,973 from negative electrode
materials having the formula
Before calculating the utilization of the tested electrodes, the
Co, Ni, and Zn concentrations present in the spherical
tri-precipitate nickel hydroxide active material and in the active
material paste (spherical nickel hydroxide+Co+CoO) were measured.
Table 1.1, below, shows the compositions of each sample. After the
addition of Co and CoO the amount of atomic Ni--. present in the
tri-precipitate decreases from 90 atomic wt % to 72 wt % in the
material used for pasting. This corresponds to about 80 wt % of
Ni(OH).sub.2 in the pasting material. The the ICP analysis (as
shown in Table 1.1) species that the amount of the spherical
tri-precipitate nickel hydroxide materials indicated only 5 wt %
Co(OH).sub.2 and 5 wt % Zn(OH).sub.2.
TABLE 1.1 ______________________________________ Atomic weight
percent of Ni, Co, and Zn found in spherical nickel hydroxide
tri-precipitate and in the active material paste. The samples were
analyzed by ICP. Ni (atomic Co (atomic Zn (atomic Material wt %) wt
%) wt %) ______________________________________ spherical nickel 90
5 5 hydroxide Tri-precipitate Active Material 72 24 4 Paste
______________________________________
Using the 80 wt % Ni(OH).sub.2 present in the active material paste
as a basis for calculation, the theoretical capacity of a 5 g
comparative pasted electrode (3 in.sup.2 area) can be calculated as
follows:
Total weight of the positive: 5.00 g
Weight of a 3 in.sup.2 foam: 0.93 g
Total weight of the powder: 5-0.93 g
Total Ni(OH).sub.2 weight: 4.07.times.0.80=3.25 g
Theoretical capacity: 3.25 g.times.0.289 Ah/g=0.94 Ah
The comparative samples were charged and discharged. A total of 6
cycles provided a stable electrode capacity. The electrodes were
formed using the following formation process. The electrodes were
charged at C40 for 24 hr followed by an additional charge of 24 hr
at C10. The electrodes were then discharged at C10. Two more cycles
were applied by charging the electrodes for 8 hrs at C10 and 9 hrs
at C20 followed by discharge at C10. The last three cycles were
performed at a C10 rate with a 110% overcharge.
It should be noted that the electrodes experienced a charge plateau
at .about.0.9 V that lasted for at least 12 hrs. This plateau has
been attributed to the formation of the cobalt conduction network
by the Co(OH).sub.2 .fwdarw.Co.sub.3 O.sub.4 and Co.sub.3 O.sub.4
.fwdarw.CoOOH oxidation reactions.
Four comparative electrodes were prepared and tested. Their
capacities are presented in table 1.2, below. These electrodes had
an average capacity of 0.87 Ah (see Table 1.2).
TABLE 1.2 ______________________________________ Electrode capacity
and utilization of four comparative pasted electrodes tested in a
tri-electrode cell. Experimental Theoretical Utilization Sample
Weight (g) Cap. (Ah) Cap. (Ah) (%)
______________________________________ C1 4.87 0.86 0.91 94 C2 5.05
0.88 0.95 93 C3 5.00 0.88 0.94 94 C4 4.89 0.85 0.92 92 Average 0.87
93 ______________________________________
Utilization was obtained by comparing the experimental capacity
with the theoretical capacity (using C3 as an example):
Utilization: 0.88/0.94=94%
The gravimetric energy density was calculated by dividing
experimental capacity by the total weight of the 3 in.sup.2
electrode (5 g excluding the electrode connector tab). The
volumetric energy density was calculated as the capacity divided by
the total volume of the electrode (1.55 cc). Again using C3 as an
example:
Gravimetric Energy Density (mAh/g): 880/5=176
Volumetric Energy Density (mAh/cc): 880/1.55=568
Finally, measurements on two comparative pasted electrodes at a
C/10 rate with 110% overcharge were performed. The 48 hr charging
period at lower currents was not used in this case. As shown in
Table 1.3 the initial capacity was quite low (0.56 Ah), but
immediately improved on subsequent cycles yielding final capacities
and utilizations slightly higher than those mentioned earlier. It
could be that the long charging period of 48 hr at low current may
not be necessary when using these cells. As judged by the plateau
at .about.1 V the formation process at a C/10 rate consumes less
charge (.about.0.2 Ah) than the electrodes formed at lower rate
(0.25 Ah). However the electrode capacity has improved.
TABLE 1.3 ______________________________________ Electrode capacity
and utilization of two comparative pasted elec- trodes tested in a
tri-electrode cell. The electrodes were formed at C/10 without the
48 hr period of charging at low current. Experimental Theoretical
Utilization Weight (g) Cap. (Ah) Cap. (Ah) (%)
______________________________________ 4.93 0.89 0.92 97 4.95 0.89
0.93 96 ______________________________________
Next, spherical nickel hydroxide powder was modified by
precipitating cobalt hydroxide from a cobalt sulfate solution onto
the nickel hydroxide particles. The resulting cobalt hydroxide
layer increased the cobalt hydroxide content from 5 to 10 wt %.
With this modified spherical powder it was possible to remove the
metallic cobalt (Co) and cobalt oxide (CoO) additives completely
from the active material paste. Utilization in these electrodes was
over 95%, specific capacity was 189 mAh/g versus 180 mAh/g in
standard comparative electrodes. This represents a 5% improvement
in specific capacity.
The present invention involves precipitating Co(OH).sub.2 on the
spherical nickel hydroxide powder before the preparation of the
active material paste. This permits the formation an interconnected
network of Co(OH).sub.2 /CoOOH on the surface of the particle
before electrode preparation in order to decrease or eliminate, the
amount of Co and CoO needed in the pasted material.
Early work with thin films indicated the advantages of having a
layer of CoOOH closer to the current collector. In this work, the
current collector was nickel or gold foil onto which the metal
hydroxide film had been deposited. Reasoning that the current
collector is the external surface of the crystallites, the current
inventors were able to conclude that a layer of Co(OH).sub.2 on
this external surface would essentially behave as a thin film.
The original spherical nickel hydroxide powder used in the
comparative pasted electrodes contained 5 wt % of Co(OH).sub.2
homogeneously distributed throughout the particles. For the present
work, additional Co(OH).sub.2 was then precipitated on these
particles from a CoSO.sub.4 solution using KOH. Continuous stirring
and dilute solutions of KOH were used to minimize large local
concentration of OH.sup.- ions. (Such local .sup.-OH concentrations
promote the formation of discrete Co(OH).sub.2 particles separate
from the spherical nickel hydroxide material.) Essentially, the
present invention uses spherical nickel hydroxide powder particles
as seeds for the formation of a new nickel hydroxide material.
The results obtained were from spherical nickel hydroxide powder
enriched in Co(OH).sub.2 from a standard value of 5 wt % (in solid
solution within the matrix of the nickel hydroxide powder
particles) to 7.6 and 10.4 wt % (which includes both the solid
solution and coating layer cobalt hydroxide). With the 7.6 wt %
powder, pasted electrodes, containing 25% and 50% less Co and CoO
than the comparative examples, were prepared. The spherical nickel
hydroxide powder enriched to 10.4 wt % cobalt hydroxide was tested
without the use of any Co or CoO additives in the pasted
electrode.
Using the same tri-electrode cell described in the comparative
examples above, the modified versions of nickel hydroxide pasted
electrodes were tested. A typical comparative baseline was prepared
by mixing spherical nickel hydroxide (5 wt % Zn(OH).sub.2, 5 wt %
Co(OH).sub.2, 90 wt % Ni(OH).sub.2) with Co and CoO. The amounts of
every component, and the total weight percent are shown in Table
2.1.
TABLE 2.1 ______________________________________ Weight and weight
percent of the components used in a compara- tive electrode. The
spherical nickel hydroxide precipitate (86.2 g) contains 5 wt %
Co(OH).sub.2 and 5 wt % Zn(OH).sub.2.
______________________________________ spherical nickel
Ni(OH).sub.2 77.58 g 80 wt % hydroxide Co(OH).sub.2 4.31 g 4.5 wt %
Tri-precipitate Zn(OH).sub.2 4.31 g 4.5 wt % Powder As Analyzed CoO
CoO 5.82 g 6.0 wt % Co Co 4.57 g 4.7 wt % Total 96.59 g
______________________________________
In the comparative examples above, a utilization of 96-97% was
obtained after six cycles at C/10 with 110% overcharge. Additional
measurements with similar electrodes have confirmed these results.
Table 2.2 shows a total of five electrodes where the average
utilization was 97%. The first two electrodes in the table were
taken from the comparative examples of table 1.2. The initial
capacity is low but improves rapidly and remains stable after six
cycles.
TABLE 2.2 ______________________________________ Electrode capacity
and utilization of comparative pasted elec- trodes tested in a
tri-electrode cell. The electrodes were formed at a C/10 rate.
Experimental Theoretical Utilization Weight (g) Cap. (Ah) Cap. (Ah)
(%) ______________________________________ 4.93 0.89 0.92 97 4.95
0.89 0.93 96 4.80 0.87 0.89 98 4.92 0.88 0.92 96 4.81 0.87 0.89 98
Average 97 ______________________________________
The theoretical capacities of the additional comparative examples
shown in the table were obtained by first calculating the total
amount of Ni(OH).sub.2 present in the electrode. For example, for
the first electrode shown in Table 2.1 the theoretical capacity was
calculated as shown below:
Total weight of the positive: 4.93 g
Weight of a 3 in.sup.2 foam: 0.93 g
Total weight of the powder: 4.93-0.93=4.00 g
The total Ni(OH).sub.2 weight was then calculated by multiplying
the total weight of the powder by 0.80 (only 80 wt % of the powder
is pure Ni(OH).sub.2),
Total Ni(OH).sub.2 weight: 4.00.times.0.80=3.2 g
and since 1 g of Ni(OH).sub.2 is equivalent to 0.289 Ah, the
capacity expected from 3.2 g was:
Theoretical capacity: 3.2 g.times.0.289 Ah/g=0.92 Ah
The utilization of the electrode was then obtained by comparing the
capacity measured in the last cycle (0.89 Ah) with the theoretical
capacity:
Utilization: 0.89/0.92=97%
The gravimetric energy density was calculated by dividing the
experimental capacity by the total weight of the 3 in.sup.2
electrode (4.93 g without the tab). The volumetric energy density
was calculated as the capacity divided by the total volume of the
electrode (1.57 cc):
Gravimetric Energy Density (mAh/g): 890/4.93=180
Volumetric Energy Density(mAh/cc): 890/1.57=565
Before the preparation of the modified electrodes the CO(C)H).sub.2
content of the spherical nickel hydroxide material was increased by
precipitating Co(OH).sub.2 from a CoSO.sub.4 solution. The
precipitation was performed by adding approximately 73 ml of 0.5 M
CoSO.sub.4 solution to 110 g of spherical nickel hydroxide powder.
This corresponds to 3.4 g of additional Co(OH).sub.2. The amount of
cobalt in the sulfate solution was calculated to theoretically
increase the total Co(OH).sub.2 content of the nickel hydroxide
particles from approximately 5 wt % to 8 wt %. Then while
continuously and vigorously stirring the solution with the added
particles, KOH 0.1N was added drop by drop until the pH increased
to 8. At that point the stirring was stopped and the powder
particles were allowed to settle at the bottom of the beaker. The
solution on top was clear and without the characteristic reddish
color of cobalt cations in solution. The precipitate was then
rinsed several times with distilled water to remove excess sulfate
and dried in the oven at 60 degree C. By ICP analysis the coated
nickel hydroxide was found to contain a total (solid solution and
coated ) of 7.6 wt % Co(OH).sub.2. The new composition of the
modified spherical nickel hydroxide material was 87.5 wt %
Ni(OH).sub.2, 4.9 wt % Zn(OH).sub.2, and 7.6 wt % Co(OH).sub.2.
Using this powder the relative ratios of the components in the
active material paste were changed by decreasing the Co and CoO
content by 25% (see Table 2.3, below). The total weight percent of
Ni(OH).sub.2 in the paste was 80 wt %, which is similar to the
nickel hydroxide content found in the comparative electrodes.
TABLE 2.3 ______________________________________ Weight and weight
percent of the components used in a modified electrode. The
spherical nickel hydroxide precipitate (91.5 g) contains 7.6 wt %
Co(OH).sub.2 and 4.9 wt % Zn(OH).sub.2. The Co and CoO content was
decreased by 25% (see Table 2.1).
______________________________________ spherical nickel
Ni(OH).sub.2 80.0 g 80 wt % hydroxide Co(OH).sub.2 6.95 g 7 wt %
tri-precipitate powder as Zn(OH).sub.2 4.48 g 4.5 wt % analyzed CoO
CoO 4.37 g 4.4 wt % Co Co 3.43 g 3.5 wt % Total 99.23 g
______________________________________
Table 2.4 shows the results obtained with two of the modified
electrodes. The utilization of 95% was very close to the values of
the comparative electrodes. The gravimetric energy densities were
also very similar. The volumetric energy density was higher since
the electrode was thinner (31 mil). The energy densities for one
electrode were:
Gravimetric Energy Density (mAh/g): 900/4.92=183
Volumetric Energy Density (mAh/cc): 900/1.525=590
TABLE 2.4 ______________________________________ Electrode Capacity
and Utilization of two modified pasted elec- trodes with 25% less
Co and CoO used as additives in the active material paste.
Experimental Theoretical Utilization Weight (g) Cap. (Ah) Cap. (Ah)
(%) ______________________________________ 4.92 0.90 0.95 95 4.94
0.90 0.95 95 Average 95 ______________________________________
During charging, of the modified electrodes, a low voltage plateau
was observed during the first cycle, but it developed for a short
time (less than two hours). This was to be expected because of the
smaller amount of Co and CoO used during electrode preparation.
Based on the capacities measured and the energy densities
calculated it can be concluded that a 25% decrease in Co and CoO
did not affect the utilization and the electrode capacity of
electrodes prepared with the modified spherical nickel hydroxide
powder. Results show that the extra Co(OH).sub.2 added was formed
on the surface of the spherical nickel hydroxide powder.
Next, using the modified spherical nickel hydroxide powder (7.6 wt
% in Co(OH).sub.2), the Co and CoO content in the paste was
decreased by 50% (as compared with the comparative examples). The
final amounts used for the preparation of the pasted electrodes are
shown in Table 2.5.
TABLE 2.5 ______________________________________ Weight and weight
percent of the components used in a modified electrode. The
modified spherical nickel hydroxide precipitate (94.1 g) contains
7.6 wt % Co(OH).sub.2. The Co and CoO content has been decreased by
approximately 50% (compare with Table 2.1).
______________________________________ spherical nickel
Ni(OH).sub.2 82.3 g 82.6 wt % hydroxide Co(OH).sub.2 7.16 g 7.2 wt
% Tri-precipitate Zn(OH).sub.2 4.61 g 4.6 wt % Powder As Analyzed
CoO CoO 2.91 g 2.92 wt % Co Co 2.58 g 2.59 wt % Total 99.59 g
______________________________________
Note that the total Ni(OH).sub.2 content in the final paste was
82.6 wt %. The utilization of the electrodes, using this paste,
changed little compared with the comparative electrodes (see, Table
2.6). The gravimetric and volumetric energy densities were also
very similar. The energy densities for one electrode are shown
below:
Gravimetric Energy Density (mAh/g): 930/5.08=183
Volumetric Energy Density (mAh/cc): 930/1.623=573
This electrode was thicker (33 mil) and therefore the volumetric
energy density Was lower.
During charging of these electrodes, a low voltage plateau was
observed during the first cycle, but it developed for a short time.
This was to be expected since less CoOOH is being formed from the
Co and CoO added to this electrode. These results indicate that a
50% decrease in Co and CoO does not effect utilization or electrode
capacity of electrodes prepared with the modified spherical nickel
hydroxide powder of the invention.
TABLE 2.6 ______________________________________ Electrode capacity
and utilization of modified pasted elec- trodes with 50% less Co
and CoO to the active material paste. Experimental Theoretical
Utilization Weight (g) Cap. (Ah) Cap. (Ah) (%)
______________________________________ 5.08 0.93 0.98 95.0 5.09
0.92 0.98 94.0 Average 94.5
______________________________________
Based on the results obtained with 25 and 50% reduction in Co and
CoO these additives were completely removed from the pasted
material and the Co(OH).sub.2 content in the modified spherical
nickel hydroxide powder was further increased. The precipitation
procedure was similar, however, the concentration of CoSO.sub.4 was
increased from 0.5 to 1.0 M. Therefore, approximately 73 ml of 1 M
solution of CoSO.sub.4 were added to 110 g of spherical nickel
hydroxide powder. This corresponded to 6.77 g of additional
Co(OH).sub.2 formed on the 110 g of the spherical nickel hydroxide
powder. ICP analysis confirmed an increase from 5 wt % (in the raw
spherical nickel hydroxide powder) to 10.4 wt % (in and around the
spherical nickel hydroxide powder). Using this powder, pasted
electrodes were prepared without using metallic Co or CoO as
additives (see Table 2.7). Note that the total Ni(OH).sub.2 content
increased to 84.7 wt %.
TABLE 2.7 ______________________________________ Weight and weight
percent of the components used in a modified electrode. Based on
ICP analysis the modified spherical nickel hydroxide precipitate
(86.01 g) contains 10.43 wt % Co(OH).sub.2, and 4.85 wt %
Zn(OH).sub.2 and 84.72 wt % Ni(OH).sub.2. Metallic Co and CoO
additives were not used in this case.
______________________________________ spherical nickel
Ni(OH).sub.2 72.86 g 84.7 wt % hydroxide Co(OH).sub.2 8.97 g 10.4
wt % Tri-precipitate Zn(OH).sub.2 4.17 g 4.8 wt % Powder As
Analyzed CoO none Co none Total 86.0 g
______________________________________
Table 2.8 shows the results obtained with four electrodes using
this 10.4% modified powder. The utilization did not decrease
compared with the comparative examples. The cycling profile of a
representative modified electrode shows that by the end of the
second cycle the electrodes achieved full capacity. Again the
overcharge potential was very similar to the comparative electrodes
and the plateau observed during the first cycle was very much
suppressed. Since there was no addition of Co or CoO this plateau
is believed to result from the Co(OH).sub.2 .fwdarw.CoOOH oxidation
reaction.
TABLE 2.8 ______________________________________ Electrode capacity
and utilization of pasted electrodes prepared with a modified
spherical nickel hydroxide powder in the absence of Co or CoO in
the active material paste. Experimental Theoretical Weight (g) Cap.
(Ah) Cap. (Ah) Utilization (%)
______________________________________ 4.46 0.86 0.86 100 4.64 0.87
0.91 96 4.60 0.86 0.90 95 4.52 0.86 0.88 98 Average 97
______________________________________
If one considers the gravimetric and volumetric energy densities
(see Table 2.9) the electrodes performed better than the
comparative electrodes.
TABLE 2.9 ______________________________________ Gravimetric and
volumetric energy density of pasted electrodes prepared with a
modified spherical nickel hydroxide powder in the absence of Co and
CoO in the active material paste. Weight Gravimetric Energy
Volumetric Energy (g) Density (mAh/g) Density (mAh/cc)
______________________________________ 4.46 193 583 4.64 187 590
4.60 187 583 4.52 190 583 Average 189 585
______________________________________
The usefulness of the CO(C)H).sub.2 precipitation is even more
evident if one compares these results with baseline measurements.
These are shown in Table 2.10. Here the Co and CoO were removed and
the electrodes prepared with standard spherical nickel hydroxide
material. As shown by the results the electrodes performed poorly
when the additives were removed. The utilization dropped to 70% and
the gravimetric energy density went down to 152 mAh/g.
TABLE 2.10 ______________________________________ Electrode
capacity and utilization of pasted electrodes with no Co or CoO
added to the active material paste. The powder used was standard
spherical nickel hydroxide. Experimental Theoretical Weight (g)
Cap. (Ah) Cap. (Ah) Utilization (%)
______________________________________ 5.37 0.82 1.15 71 5.36 0.80
1.15 69 Average 70 ______________________________________
Based on experiments performed on tri-electrode cells it can be
concluded that the cobalt and the cobalt oxide additives can be
significantly reduced from pasted electrodes after modifying the
standard spherical nickel hydroxide powder. This modification
involves the chemical precipitation of cobalt hydroxide on the
spherical nickel hydroxide material. By using this technique, the
cobalt hydroxide content was increased from 5 wt % to 10 wt %. With
the 10 wt % enriched powder the cobalt and the cobalt oxide can be
completely removed from the electrodes without affecting the
electrode performance.
Next, spherical nickel hydroxide powder was modified by
precipitating cobalt hydroxide from a cobalt sulfate solution. This
resulted in a cobalt hydroxide layer where the cobalt hydroxide
content was increased from 5 wt % to 7.7 wt %, 10 wt % and 12.7 wt
%. With this modified spherical nickel hydroxide powder, electrodes
were prepared without added metallic cobalt and cobalt oxide.
Measurements performed with tri-electrode cells, at a rate of
C/10,have demonstrated that the best results were obtained with
electrodes prepared with 10 wt % and 12.7 wt % cobalt hydroxide.
The utilization of these electrodes was over 96%. The specific
capacity was 187-188 mAh/g versus 178-180 mAh/g observed in
comparative electrodes.
In these experiments additional results with pasted electrodes
enriched in cobalt hydroxide from a standard value of 5 wt % to
7.7, 10, and 12.7 wt % in the absence of cobalt and cobalt oxide in
the active material paste are presented. The experiments disclosed
below were performed at a charging rate of C/10.
Baseline measurements with comparative positive electrodes were
performed using the tri-electrode cells described above. The
electrodes were prepared with cobalt and cobalt oxide additives
where the standard total nickel hydroxide content was 80 wt %. The
results obtained at C/10 were comparable to the previous data. As
shown in Table 3.1 the utilization and energy density were 94.5%
and 178.5 mAh/g, respectively, after six cycles. In the previous
examples a utilization of 97% after six cycles at C/10 with 110%
overcharge was achieved.
TABLE 3.1
__________________________________________________________________________
Weight, electrode capacity, energy density, and utilization of
comparative pasted electrodes prepared with spherical nickel
hydroxide in the presence of Co and CoO added to the active
material paste (80 wt % in nickel hydroxide). The electrodes were
formed and tested at C/10. Weight Experimental Theoretical
Gravimetric Energy Utilization (g) Capacity (Ah) Capacity (Ah)
Density (mAh/g) (%)
__________________________________________________________________________
5.11 0.91 0.97 178 94 5.09 0.91 0.96 179 95 Average 178.5 94.5
__________________________________________________________________________
Based on previous results, pasted electrodes with increased amounts
of precipitated cobalt hydroxide were prepared. Experiments were
performed with spherical nickel hydroxide tri-precipitate enriched
in cobalt hydroxide to 7.7 wt %, 10 wt % and 12.7 wt % in the
absence of cobalt and cobalt oxide in the active material paste.
The total cobalt hydroxide content of the unmodified spherical
nickel hydroxide tri-precipitate was initially 5 wt %. The
precipitation procedure was similar to the one described above: 1M
KOH was added drop by drop to a solution containing cobalt sulfate
and spherical nickel hydroxide powder. After precipitation of the
cobalt, the modified spherical nickel hydroxide was filtered,
rinsed, dried, and analyzed by ICP. The amount of cobalt hydroxide
incorporated into the final precipitate was adjusted by changing
the volume of cobalt sulfate added initially to the spherical
nickel hydroxide powder. This additional cobalt hydroxide is formed
on the surface of the spherical nickel hydroxide powder
particles.
Table 3.2 shows the results obtained with four electrodes prepared
with this modified spherical nickel hydroxide enriched in cobalt
hydroxide to 7.7 wt % (again, this number includes the initial 5%
cobalt in solid solution with the nickel hydroxide). The pasted
electrodes were prepared in the absence of cobalt and cobalt oxide
additives to the paste. At C/10 the utilization decreased from the
standard value of 94.5%, shown in the previous section, to 88%.
This drop in capacity most probably corresponds to a lower
electronic conductivity between the particles since the amount of
cobalt hydroxide used in this electrode was only 2.7 wt % above the
standard 5 wt % contained in the triprecipitate. At C/2 the
utilization decreased to 55% probably for the same reason.
TABLE 3.2
__________________________________________________________________________
Weight, electrode capacity, energy density, and utilization of
pasted electrodes prepared with modified spherical nickel hydroxide
(7.7 wt % Co(OH).sub.2, 87.4 wt % Ni(OH).sub.2, 4.9 wt %
Zn(OH).sub.2) in the absence of Co or CoO added to the active
material paste. The electrodes were formed and tested at C/10.
Weight Experimental Theoretical Gravimetric Energy Utilization (g)
Capacity (Ah) Capacity (Ah) Density (mAh/g) (%)
__________________________________________________________________________
4.92 0.90 1.00 183 90 5.03 0.93 1.04 185 89 4.99 0.89 1.03 178 86
5.05 0.92 1.04 182 88 Average 182 88
__________________________________________________________________________
Table 3.3 shows the results obtained with two electrodes prepared
with modified spherical nickel hydroxide enriched to 10 wt % in
cobalt hydroxide. Again, the electrodes were prepared in the
absence of Co and CoO in the active material paste. At C/10 the
utilization was close to the values obtained for the comparative
electrodes. The energy density, however, was higher by 4 to 5%. The
better performance of these electrodes (with a higher cobalt
hydroxide content) agree with the postulated mechanism of
conduction mentioned earlier.
TABLE 3.3
__________________________________________________________________________
Weight, electrode capacity, energy density, and utilization of
pasted electrodes prepared with modified spherical nickel hydroxide
(10 wt % Co(OH).sub.2, 85 wt % Ni(OH).sub.2, 5 wt % Zn(OH).sub.2)
without Co or CoO added to the active material paste. The
electrodes were formed and tested at C/10. Weight Experimental
Theoretical Gravimetric Energy Utilization (g) Capacity (Ah)
Capacity (Ah) Density (mAh/g) (%)
__________________________________________________________________________
4.66 0.87 0.92 187 95 4.65 0.87 0.91 187 96 Average 187 95.5
__________________________________________________________________________
Table 3.4 shows the results obtained with four electrodes prepared
with modified spherical nickel hydroxide enriched to 12.7 wt %
cobalt hydroxide. Once again, the electrodes were prepared in the
absence of Co and CoO in the active material paste. At C/10 the
energy densities were very similar to the values measured with the
electrodes describe above. The utilization, however, was higher
(99%). Preliminary results at C/2 have show only a slight decrease
in utilization (96%).
TABLE 3.4
__________________________________________________________________________
Weight, electrode capacity, energy density, and utilization of
pasted electrodes prepared with modified spherical nickel hydroxide
(12.7 wt % Co(OH).sub.2, 83 wt % Ni(OH).sub.2, 4.3 wt %
Zn(OH).sub.2) in the absence of Co or CoO added to the active
material paste. The electrodes were formed and tested at C/10.
Weight Experimental Theoretical Gravimetric Energy Utilization (g)
Capacity (Ah) Capacity (Ah) Density (mAh/g) (%)
__________________________________________________________________________
4.50 0.84 0.86 187 98 4.45 0.84 0.84 189 100 4.50 0.83 0.86 184 97
4.45 0.85 0.84 191 101 Average 188 99
__________________________________________________________________________
Table 3.5 shows the results obtained in C-cells that embody the
present invention. These C-cells were prepared using a nickel
hydroxide positive electrode materials that contained modified
spherical nickel hydroxide enriched to 10% cobalt hydroxide, 5 wt %
cobalt powder, 3 wt % CoO, and 0.3 wt % PVA binder. Control C-cells
were fabricated using standard nickel hydroxide positive electrode
material. At C/2 the internal cell pressure during cycling was
markedly better than the internal pressure of the control
cells.
TABLE 3.5
__________________________________________________________________________
Internal pressure (PSI) in C-cells of the present invention
prepared with modified spherical triprecipitate nickel hydroxide
containing 10 wt % Co(OH).sub.2, 5 wt % Zn(OH).sub.2, and 85 wt %
Ni(OH).sub.2 ; as additives 5 wt % Co and 5 wt % CoO; and 3 wt %
PVA binder. The electrodes were formed and tested at C/2. Cycles 1
50 100 150 200 250 300 350 400 450 500
__________________________________________________________________________
control 180 250 320 360 380 -- -- -- -- -- -- embodiment 100 120
130 150 170 170 180 170 170 150 150
__________________________________________________________________________
Based on examples above, it can be concluded that the cobalt and
the cobalt oxide additives can be significantly decreased in pasted
nickel hydroxide electrodes after modifying the standard spherical
nickel hydroxide powder. The modification involves the chemical
precipitation of cobalt hydroxide onto the powder particles of
spherical nickel hydroxide material. Using this method the cobalt
hydroxide content was increased from 5 wt % to 7.7 wt %, 10 wt %
and 12.7 wt %. Only with the electrodes made with 10 and 12.7 wt %
cobalt enrichment could be produced with no cobalt and cobalt oxide
additives without affecting the electrode capacity at C/10.
In the above examples, modified nickel hydroxide particles were
prepared by precipitation of divalent cobalt hydroxide onto nickel
hydroxide particles. The nickel hydroxide particles were immersed
into cobalt sulfate solution. Potassium hydroxide solution was
slowly added to the stirred solution to precipitate divalent cobalt
hydroxide onto the suspended nickel hydroxide particles. This
provided an encapsulation of the particles with cobalt
hydroxide.
In some cases, the beneficial effects of the cobalt hydroxide
encapsulation can be interfered with when there is opportunity to
dissolve the cobalt hydroxide into the battery electrolyte prior to
the initial battery charge. For example, during high temperature
heat treatment of nickel metal hydride cells prior to the first
formation charge, dissolution and migration of the cobalt hydroxide
encapsulate can result in substantial capacity loss. To overcome
this, heat treatment can be avoided. Alternatively, a more stable
form of the cobalt hydroxide encapsulate can be prepared.
By a simple modification of the above process to encapsulate the
nickel hydroxide particles with divalent cobalt hydroxide, the
encapsulate can be converted to the more stable trivalent cobalt
oxyhydroxide form. After completion of the precipitation of the
cobalt hydroxide, additional 0.1N KOH is added dropwise until the
pH is shifted from 8 to 14. This shifts the oxidation potential of
the divalent cobalt so that it can be rapidly oxidized by oxygen.
The resulting alkaline suspension of modified nickel hydroxide is
then stirred in air over night to facilitate air oxidation of the
cobalt hydroxide coating. Completion of the oxidation is evident as
the modified nickel hydroxide turns from light green to dark brown
in color. There is no oxidation of the underlying nickel hydroxide
particles because nickel hydroxide is oxidized at a higher
potential than cobalt hydroxide. The resulting suspension is
carefully filtered, rinsed to remove excess alkalinity, and dried
prior to using the modified nickel hydroxide powder to prepare
electrodes. The cobalt oxyhydroxide encapsulate is more stable than
the divalent cobalt hydroxide encapsulate due to the much lower
solubility of the cobalt oxyhydroxide. The cobalt oxyhydroxide
encapsulate is more generally useful since it does not suffer
significant dissolution even during an extended alkaline heat
treatment.
Experiments were performed using spherical nickel hydroxide
encapsulated with cobalt oxyhydroxide as described above. The
nickel hydroxide was enriched with 5 wt % additional cobalt
oxyhydroxide encapsulation. This material was mixed with 3 wt % CoO
and 5 wt % Co powder to produce pasted nickel hydroxide electrodes.
Tests in trielectrode cells indicated a gravimetric energy density
of 185 mAh/g. Tests in c-cells indicated 176 mAh/g showing a 3%
improvement over baseline results.
Another way to produce nickel hydroxide particles with a stable
cobalt oxyhydroxide encapsulate is by reacting chemically oxidized
nickel hydroxide particles suspended in water with cobalt metal
powder. Nickel hydroxide powder is oxidized with sodium
hypochlorite. It is then rinsed and dried. It is mixed with about
10 wt % cobalt powder. After an induction period, the nickel
oxyhydroxide will oxidize the cobalt metal to cobalt oxyhydroxide
in an exothermic reaction. An excellent encapsulated coating of
cobalt oxyhydroxide results.
It is also not necessary that the conductive coating of the present
invention be limited to cobalt hydroxide or oxyhydroxide. The
conductivity properties of cobalt oxyhydroxide are shared by other
higher metallic hydroxides and oxides. For example, gamma-manganese
dioxide has substantial conductivity, stays oxidized throughout the
range of operation of the nickel hydroxide electrode, and has a low
solubility.
While the modification of nickel hydroxide is described in relation
to spherical nickel hydroxide material (a 5 wt % Zn(OH).sub.2, 5 wt
% Co(OH).sub.2, 90 wt % Ni(OH).sub.2 solid solution material), the
methods and products of the present invention are equally
applicable to the nickel hydroxide solid solution active materials
of the parent application.
The improved capacity of the nickel hydroxide materials of the
parent application relates to the material's multiphase disordered
structure, .gamma.-phase stability, multiple electron transfer
capabilities, increased conductivity, and their interaction with
unique formulated electrolytes. While each of these characteristics
are discussed separately, it is believed that they are all
interrelated.
The formation of .gamma.-phase material is desirable because
.gamma.-phase material is capable of multiple electron transfers.
Higher capacity batteries using .gamma.-phase materials have, up
until now, not been possible because .gamma.-phase material could
not be stabilized.
In prior art nickel hydroxide solid solution materials, cobalt was
added to improve stability and encourage multiple electron
transfer. It was theorized that cobalt stabilized .gamma.-phase
materials because its presence creates excess positive charge in
the nickel hydroxide plates that results in the intercalation of
anions, such as CO.sub.3 .sup.2-, and water molecules between the
plates to compensate for excess positive charge. In such material,
fractionally more than one electrode is transferred. However, this
effect is short lived.
The positive electrode material described in U.S. Pat. No.
5,344,728 (the grand parent of the present application) is a
disordered active material consisting of a 10% coprecipitated
cobalt active material with layers of enriched cobalt substituted
on the electrode surface. This material contains a nominal
percentage of stabilized .gamma.-phase material as a result of its
disordered microstructure. Building on this work with disordered
nickel hydroxide materials, it was found that predominantly
.gamma.-phase nickel hydroxide materials that are multi-phased
could be produced and the stability of the .gamma.-phase of these
materials could be significantly improved. The nickel hydroxide
positive electrode materials of the parent application, U.S. Pat.
No. 5,348,822, because of their disordered nature, exhibit stable
multiple electron transfer.
These materials also exhibit density changes that result in a
higher surface area such that the electrolyte reactants within the
nickel hydroxide have better catalysis, in addition, the
conductivity is improved by the formation of filamentous conductive
regions that extend from areas of high conductivity immediately
adjacent to the nickel current collector to the exterior of
individual nickel hydroxide particles. Thus, nickel hydroxide
electrodes of the parent application have increased conductivity
between the active material and the nickel current collector
independent from the conductive cobalt hydroxide coatings of the
present invention.
The disordered materials of the parent application are
compositionally and/or structurally disordered. "Compositionally
disordered" as used herein is specifically defined to mean that
this material contains at least one compositional modifier and/or a
chemical modifier. The at least one compositional modifier may be a
metal, a metallic oxide, a metallic oxide alloy, a metal hydride,
and/or a metal hydride alloy. Preferably, the compositional
modifier is chosen from the group consisting of Al, Bi, Co, Cr, Cu,
Fe, In, La, Mn, Ru, Sb, Sn, Ti, and Zn. The chemical modifier is
chosen from the group consisting of Al, Ba, Ca, Co, Cr, Cu, F, Fe,
K, Li, Mg, Mn, Na, St, and Zn.
"Structurally disordered" as used herein is specifically defined to
mean having a more conductive surface, filamentous regions of
higher conductivity, and multiple or mixed phases where .alpha.,
.beta., and .gamma.-phase regions may exist individually or in
combination. The disordered materials of the parent application
contain 8 to 30 atomic percent; preferable 10 to 20 atomic percent
of at least one of the compositional modifiers or chemical
modifiers described above. Materials of the parent application are
formed when a compositional modifier is incorporated into the
material itself, as distinguished from the cobalt hydroxide formed
as a layer upon the particles of the material, which increases
conductivity of the active material. These compositional modifiers
tend to disrupt the formation of large crystallites which can lead
to higher resistance materials. The increased disorder due to
smaller crystallites tends to provide electronic conductivity of
the bulk active material not present in more crystalline forms.
Further, the local disorder caused by distortions surrounding these
modifiers has a similar effect. These materials can also be formed
through charge and discharge treatments, particularly pulsed
charging/discharging that encourage disorder, the formation of
microcracks, and a reduction in particle size.
In order to form disordered materials containing 8 to 30 atomic
percent chemical and compositional modifiers, several processing
variations may be utilized including coprecipitation of any number
of compositional modifiers in a chemical conversion impregnation or
electrochemical impregnation process, including that of high
density, spherical type materials. These active materials may be
used in all types of nickel battery positive electrodes including
sintered electrodes, foam type pasted electrodes and fiber type
pasted electrodes. The modifiers may be added to conversion
electrolytes during impregnation, formation, or activation, or
directly to the electrolyte in a sealed or vented cell.
The disordered materials are multiphase polycrystalline materials
having at least one .gamma.-phase that contains compositional
modifiers or combinations of compositional and chemical modifiers
that promote the multiphase structure and the presence of
.gamma.-phase materials. These compositional modifiers are chosen
from the group consisting of Al, Bi, Co, Cr, Cu, Fe, In, La, Mn,
Ru, Sb, Sn, Ti, and Zn. Preferably, at least 3 compositional
modifiers are used.
As a result of their disordered structure and improved
conductivity, these materials do not have distinct oxidation states
such as 2.sup.+, 3.sup.+, or 4.sup.+. Rather, these materials form
graded systems that pass 1.2 to 2 electrons.
The materials of the present invention are also distinguished over
the prior art by the non-substitutional incorporation of at least
one chemical modifier around the plates of the nickel hydroxide
electrode material. The phrase "non-substitutional incorporation
around the plates", as used herein means the incorporation into
interlamellar sites or at edges of plates. These chemical modifiers
are preferably chosen from the group consisting of AI, Ba, Ca, Co,
Cr, Cu, F, Fe, K, Li, Mg, Mn, Na, Sr, and Zn.
Contrary to the prior art, these nickel hydroxide positive
electrode materials are disordered materials. The use of disordered
materials permits permanent alteration of the properties of the
material by engineering the local and intermediate range order. The
general principals of this are discussed in U.S. Pat. No.
4,623,597, the contents of which are hereby incorporated by
reference. These disordered nickel hydroxide positive electrode
materials are multiphase materials having a polycrystalline
.gamma.-phase that can additionally contain at least one structure
selected from the group consisting of (i) amorphous; (ii)
microcrystalline; (iii) polycrystalline lacking long range
compositional order with three or more .gamma.-phases of said
polycrystalline structure; and (iv) any combination of said
amorphous, microcrystalline, or polycrystalline structures.
Another reason for the improved performance of the nickel hydroxide
materials of the present invention is that the chemical modifiers
provide for electronic overlap between adjacent nickel hydroxide
plates thereby increasing the inherent conductivity of the nickel
hydroxide material. This latter possibility was considered
previously (see, Corrigan, et al., et al, 90-4 Proceedings of the
Symposium on Nickel Hydroxide Materials 97 (1990). However, the
prior art does not teach that major gains in specific capacity can
be achieved by the incorporation of chemical modifiers between
plates of disordered material such that these chemical modifiers
provide electronic overlap through spatially extended d-orbitals as
in the present invention.
Compositional modifiers are incorporated into the nickel hydroxide
electrode material using, for example, conventional precipitation
procedures. Electrolyte ions can be incorporated into the
interlamellar regions, for example, during oxidation in alkaline
electrolyte solution. Chemical modifiers can be incorporated into
non-substitutional sites in the interlamellar regions, for example,
by treatment of oxidized nickel hydroxide materials with salt
solutions. The incorporation of combinations of compositional
modifiers, electrolyte ions, and chemical modifiers are believed to
be especially useful.
In one method, oxidized nickel hydroxide is treated with metal
nitrate salt solution and with metal hydroxides then precipitated
by cathodic deposition from this nitrate solution. In another
method, the oxidized nickel hydroxide is treated with metal salt
solution with metal hydroxide and then precipitated by subsequent
treatment with alkaline solution. Oxidized nickel hydroxide
material could be prepared by electrochemical oxidation in alkaline
solution or by treatment with a suitable chemical oxidant such as
hydrogen peroxide or sodium hypochlorite.
The choice of disordered materials has fundamental scientific
advantages: as seen, a substantial number of elements can be
included in the lists of modifiers. These elements offer a variety
of bonding possibilities due to the multi-directionality of
d-orbitals. The multi-directionality of d-orbitals provides for a
tremendous increase in density. A considerable increase in electron
transfer capacity is possible in the disordered alloys compared to
crystalline structures of the prior art. The preparation of
disordered alloys produces large numbers of grain boundaries and a
large surface area leading to the increased conductivity and
hydrogen diffusion, and subsequently, multiple electron transfer of
the materials of the present invention. Thus, in addition to
compositional disorder, there occurs topological disorder at phase
boundaries of the multi-phase alloy. This increases enormously the
density of catalytic sites.
The material has been observed to transfer up to 1.52 electrons per
atom during reversible cycling. Cycling tests indicate that
multiple electron transfers remain stable throughout the life of
the cell. Thus, it is expected that cells fabricated using these
materials would exhibit excellent capacity throughout their
lives.
These materials can be prepared in some circumstances by first
oxidizing the nickel hydroxide electrode material so that many of
the nickel ions are in the 3+ state. The nickel hydroxide electrode
material is then treated with a cation solution, such as by
dipping, rinsing, or spraying. The treated material is then
reduced, triggering the reaction shown in this equation (where M is
a metal ion):
As a result of this reaction, chemical modifiers are
non-substitutionally incorporated around the plates of the nickel
hydroxide electrode material. This reaction can be accomplished
electrochemically or chemically. A chemical method, for example
could be accomplished by placing electrode powder in an oxidizing
solution, treating the oxidized powder with a cation solution, and
triggering the oxidation of the treated powder using hot water. The
resulting powder could then be pasted onto a foamed nickel
substrate. An electrochemical method, could be accomplished by
oxidizing formed nickel hydroxide material electrochemically,
dipping the oxidized material in a cation solution, and using a
current to trigger the oxidation reaction. Variations of these
methods such as a chemical oxidation and an electrochemical
reduction or a electrochemical reduction and a chemical reduction
are alternative methods.
Other methods of preparing the disordered materials are activation
methods that involve a 200-300% increase in current density, a
pulsed or intermittent charge/discharge treatment, or both
increased current density and a pulsed treatment. Nickel hydroxide
positive electrode materials produced by these methods have a
capacity greater than the 289 mAh/g theoretical capacity
considering only single electron transfer.
Additional improvement of the disordered material are possible when
these disordered materials are combined with electrolytes where the
electrolyte comprises at least one element chosen from the group
consisting of Ba, Ca, Cs, K, Na, Ra, Rb, and St, combined with at
least one member of the group consisting of Br, CI, F, OH.
Particular examples of such electrolytes are formulations of KOH
and CsF and KOH and CsOH.
It is obvious to those skilled in the art that these positive
electrode materials may be prepared by additional methods without
departing from spirit and scope of the present invention.
The drawings, discussion, descriptions, and examples of this
specification are merely illustrative of particular embodiments of
the invention and are not meant as limitations upon its practice.
In particular, Ni--Cd and Ni--MH cells are specifically discussed,
however, the positive electrodes of the present invention can be
used with any Ni based negative cell, such as NiZn and NiFe. Thus,
it is the following claims, including all equivalents, that define
the scope of the invention.
* * * * *